1. Field of the Invention
The present invention relates to a drive control technology of a stepper motor used to drive the indicator needle of an indicator needle type analog meter or the like, and relates especially to a technology that prevents loss of synchronism.
2. Description of the Related Art
Conventionally, an analog type meter apparatus that rotates an indicator needle by a stepper motor is widely known as a vehicle speed meter or the like. This meter apparatus includes a stepper motor control device. The stepper motor control device includes a sensor that detects physical quantities (vehicle speed) and a control device that calculates an indicating angle of the indicator needle based on inputs, that is, data corresponding to the physical quantities (vehicle speed) sent from a sensor and outputs the indicating angle. The control device implements a target rotational angle computation process that calculates a target rotational angle based on data corresponding to the physical quantities (vehicle speed) sent from the sensor, that is, a rotational angle of a drive shaft of the stepper motor necessary for obtaining the indicating angle, a target rotational angle change computation process that calculates a target rotational angle change which is a difference between a present rotational angle of the drive shaft of the stepper motor and the target rotational angle, and a divisional target rotational angle computation process that equally divides the target rotational angle change by a first divisional target rotational angle within a range not exceeding a rotational limit for loss of synchronism of the stepper motor. The control device is a drive control device of the stepper motor that drives the stepper motor by supplying to the stepper motor a second divisional target rotational angle which is a value calculated by the divisional target rotational angle computation process as a rotational angle per one control period.
Loss of synchronism, that is, misalignment generated between the actual indicating angle of the indicator needle and an indicating angle controlled by the drive control device for a stepper motor is problematic in a drive control device for a stepper motor as such.
One of the reasons for this loss of synchronism is that in the case if rotational angle changes of a drive shaft are too large, influences due to the inertial force of the drive shaft become large so that there are cases in which synchronization is difficult because of the differences generated between the drive shaft and the indicating angle.
A drive control device for a stepper motor described in, for example, JP2004-328807A is known to prevent loss of synchronism in such cases of excessive rotational angle.
This conventional drive control device for a stepper motor sets a limit value to a rotational angle of the drive shaft in a single control period and prevents an angle rotation surpassing the limit value during a single control period. Thereby loss of synchronism due to the above described excessive rotational angle changes of the drive shaft is prevented.